Disc drives are used as primary data storage devices in modern computer systems and networks. A typical disc drive comprises one or more rigid magnetic storage discs which are arranged about a spindle motor for rotation at a constant high speed. An array of read/write heads are provided to transfer data between tracks defined on the disc surfaces and a host computer in which the disc drive is mounted.
The heads are mounted to a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop servo control system. The actuator includes an actuator motor (such as a voice coil motor, VCM) and one or more actuator arms which support the heads over the disc surfaces. The servo control system applies currents to the VCM to move the heads in response to detected and estimated positions of the heads as well as command inputs indicating desired positions of the heads.
The servo control system operates in two primary modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head toward the destination track. For relatively longer seeks, a velocity control approach is used whereby the velocity of the head is repeatedly determined and compared to a velocity profile defining a desired velocity trajectory for the head. Corrections to the amount of current applied to the VCM during the seek are made in relation to velocity error (i.e., the difference between actual and desired velocity).
At such time that the head reaches a predetermined distance away from the destination track (such as one or more tracks away), the servo control system transitions to a settling mode wherein the head is settled onto the destination track. Thereafter, the servo control system enters a track following mode wherein the head is caused to follow the destination track until the next seek operation is performed. Disc drive designs thus typically use proximate time optimal control with a velocity profile to control a head during a seek, a state estimator based controller with relatively slow integration to settle the head onto the destination track, and the same state estimator based controller with relatively fast integration for track following.
A problem that can arise during a seek is oscillation of the head caused by resonance mode excitation of the actuator arm. The abrupt application of current to the VCM to quickly accelerate and decelerate the heads provides broad spectrum excitation of the actuator; depending upon various factors, such as seek length, a particular seek operation may result in the excitation of an actuator arm at a particular resonant frequency (e.g., 800 Hz) with significant amplitude or phase characteristics that cannot be readily rejected by the servo loop. Such oscillation can undesirably extend the total seek time, adversely affecting data transfer rate performance.
Prior art approaches to reducing the effects of such excitations have included modification of the velocity profile to apply current transition shaping, such as proposed by U.S. Pat. No. 4,965,501 issued to Hashimoto; modification of the physical actuator assembly structure to change the resonant frequencies to levels that can be better compensated by the controller, such as proposed by U.S. Pat. No. 5,801,905 issued to Schirle et al.; and the provision of an adaptive table to provide compensation values that are applied during settling mode, such as proposed by U.S. Pat. No. 6,166,876 issued to Liu.
While operable, there are limitations associated with these and other prior art approaches. Changing the current profile to reduce excitation can degrade servo performance by increasing acceleration and deceleration times. Modifying the physical actuator assembly structure to change the mechanical response can be costly and does not lend itself to individual tuning for different electrical offset/mechanical tolerance combinations present within a population of drives during manufacturing. The use of adaptive tables resident in memory imposes a latency cost to access and retrieve a value at each servo interrupt, and this cost becomes increasingly burdensome at higher servo sample rates and with the compensation of larger numbers of harmonics.
Accordingly, there remains a continued need for improvements in the art to compensate for actuator arm oscillation during settling mode, and it is to such improvements that the present invention is directed.